Fiber optic sensors are known that use light energy and optical fibers to sense different physical parameters such as pressure, temperature, acceleration etc. Most of them consist of light source, photo detector, one or few optical fibers, reflective target and a sensitive to a certain physical effects element someway attached to the optical fibers or reflective target. Via a transmitting optical fiber light from a light source is dispatched to reflective target that partly reflects it back through a receiving optical fiber to a photo detector. Under a certain physical effects a sensitive element changes the relative position of the optical fiber and reflective target thereby changing the intensity of light reflected by the target into receiving optical fiber and transformed by the photo detector into electrical signal. Some of the fiber optic sensors include only one optical fiber combining transmitting and receiving fibers in one. Examples of such sensors are disclosed in U.S. 2009/0123112, U.S. 2007/0247613 and U.S. Pat. No. 5,771,091.
The reflective target is the most exacting and thus most expensive part of these sensors. Even small distortions of its shape or degradation of its reflective surfaces caused by temperature variations can dramatically deteriorate the sensor characteristics.
U.S. Pat. No. 4,915,882 discloses a method for manufacturing uniformly smooth monocrystal reflectors of copper, silver or gold using a crucible polished to optical quality on the surface in contact with the reflecting surface of the monocrystal. It is noted that monocrystal reflectors withstand much better the extreme thermal loads caused by laser beams, but that the advantages inherent in the monocrystalline structure of the reflecting metal body are partially lost again during forming and/or machining as these operations modify the homogeneous crystalline texture. Reflection produced by monocrystal reflectors was found to be better for etched surfaces than in polished surfaces. Etched surfaces, however, are nonhomogeneous so that while of interest as protective shields against laser beams they do not lend themselves to optical or similar purposes in which a specific optical path requires a precisely defined reflecting surface.
U.S. Pat. No. 4,414,471 discloses sensing of acoustic waves achieved by providing spaced apart stationary and cantilevered optic fibers whereby inertial forces created by acoustic signals modulate an optical signal carried by the fibers through vibration of the cantilevered fiber. In one embodiment, the sensor includes a cantilevered beam mounted at the far end to a rigid structure and having a reflective member such as a concave mirror at the free end thereof. The end of optical fiber is disposed at the center of the sphere of which the mirror surface is a section. Light fed into the fiber is reflected from the mirror, received by the fiber and applied to a detector at. When acoustic waves are incident on the transducer they will cause vibration of the cantilevered beam due to inertial forces. The mirror attached to the beam also vibrates and amplitude modulates the light received by the mirror and returned to the fiber.
Our co-pending U.S. Ser. No. 13/935,955, whose contents are wholly incorporated herein by reference, discloses a fiber optic accelerometer comprising a cantilever section which moves upon vibration or acceleration of the accelerometer such that its position relative to a reflective target changes thereby reducing the instantaneous intensity of light reflected by the target into the second end of the optical fiber and measured by the photo detector. The reflective target is formed of an optical fiber stub having a first end proximate the free second end of the optical fiber and a second end remote therefrom.
In one embodiment, the first end of the optical fiber stub has a slanted surface formed at an angle to an optical axis of the optical fiber stub and the second end of the optical fiber stub is cut perpendicularly to the optical axis and is coated with a highly polished efficient light reflecting material.
In another embodiment, the first end of the optical fiber stub has a stepped cut so as to present a first surface portion closer to the end of the optical fiber and a more distant second surface section and the second end of the optical fiber stub is cut perpendicularly to said optical axis and is coated with a highly polished efficient light reflecting material.
In both cases, there is no change in reflectivity of the optical fiber stub, the variation in signal injected into the optical fiber being caused solely by the off-axis reflection of light from the optical fiber stub owing to the deflection of the cantilever such that movement of the free end of the optical fiber causes a lessor or greater amount of the reflected light to be captured by the free end of the optical fiber. The same is true in U.S. Pat. No. 4,414,471 where the concave mirror reflects light into the cantilever regardless of its deflection, the vibration of the mirror serving to modulate the light prior to its reflection into the free end of the fiber.